Contact Lens Update

Lyndon Jones, PhD, FCOptom, DipCLP, DipOrth is a professor at the School of Optometry and Vision Science and Director of the Centre for Contact Lens Research at the University of Waterloo.

A major impediment to the growth of the contact lens market remains the fact that contact lenses induce end-of-day dryness symptoms in some 50% of wearers. ^{1, 2} This results in poor comfort for the last 2-3 hours of the day in a substantial number of people ^{2, 3} and is a major driving force behind contact lens dropout, ^{4, 5} which occurs in some 20-25% of all patients who commence lens wear. ^{5, 6} Despite the introduction of many new contact lens materials and solutions over the past decade, the percentage of wearers who complain of end-of-day dryness and discomfort has hardly changed in 20 years.

Corneal oxygenation: An issue that has been addressed
While contact lens discomfort remains an issue, one issue that has been addressed relates to corneal oxygenation. Historically, the relatively poor oxygen transmission of the initial low water content hydrogel materials resulted in many wearers exhibiting a variety of physiological issues, particularly those who wore their lenses for long periods of time or who had relatively thick lenses due to a high spherical prescription or being a toric design. This was exacerbated in people who wore lenses overnight and many reports occurred of limbal hyperemia, stromal striae, neovascularisation, epithelial microcysts and endothelial changes, as described in various reviews. ^7-10 To overcome this, companies developed lenses with higher water content, which increased the oxygen permeability (Dk) of the lens material, ^11-13 but not to the level of preventing significant overnight corneal swelling. It soon became apparent that such materials dehydrated if their thickness was below a minimal level, resulting in marked corneal staining. ^{14, 15} Commercially viable high water content lenses needed to be made thicker, which mitigated the dehydration issues but negated some of the oxygen transport benefits associated with increasing the water content.

Thus, it transpired that using conventional hydrogel lens materials, in which Dk is proportional to water content, resulted in oxygen transmissibility (Dk/t) values that were limited by the thickness of the lens materials and resulted in materials with Dk/t values in the high teens to mid 20s. ^{16, 17} However, these high water content hydrogels were clinically successful and did minimise hypoxic clinical signs in the majority of subjects who wore their lenses on a daily wear basis. ^18-21

The introduction of frequent replacement and disposable lenses
Shortly after their widespread adoption, it became apparent that high water content hydrogels were more likely to deposit with tear film components ^22-26 and that their life-span was reduced compared with lower content hydrogels. ²⁷ This, along with the development of sophisticated manufacturing methods, largely drove the introduction of frequent replacement and disposable systems and the late 1980s and early 1990s were dominated by the use of mid- or high water content materials that were disposed of between one day and one month.

Silicone hydrogel materials: Increased oxygen to the cornea
The late 1990s saw the commercialisation of silicone hydrogel (SiHy) materials, in an effort to provide increased oxygenation to the cornea and permit safe, overnight wear. These materials provide greatly improved Dk/t to the cornea and have largely eliminated hypoxic signs, whether worn on a daily wear or overnight basis. ^{17, 28-31}

These materials have shown steady market growth and now account for over 60% of all new fits in many markets worldwide. ³² However, examination of this data also shows that many practitioners are still opting to fit hydrogel materials, despite the widespread availability of silicone hydrogels for over a decade, or opting to leave wearers in conventional hydrogels and not ‘upgrade’ them to modern materials. Why is this?

Still fitting hydrogels: A matter of comfort?
SiHys are generally more hydrophobic than hydrogels due to the incorporation of various siloxane components. ^33-37 This produces issues with increased deposition of certain lipids on some SiHy materials, ^38-42 increased denaturation of deposited protein ^43-45 and some wettability issues in certain wearers. Anecdotally, these issues appear worse with those wearers who have marginal tear films and excess lipid contamination of the tears, possibly due to meibomian gland dysfunction, ⁴⁶ although this remains to be confirmed. However, many practitioners have certainly experienced situations in which they have transitioned a patient from successfully wearing a hydrogel material into a SiHy and been unable to obtain similar levels of comfort, wettability or wearing time, resulting in them returning to a hydrogel material.

So, has the contact lens industry “given up” on hydrogel materials? A significant proportion of the contact-lens wearing population continues to wear hydrogel lenses, particularly among wearers of daily disposable lenses, which have only relatively recently become widely available in SiHy materials. ⁴⁷ In some countries, notably in northern Europe and Japan, daily disposables are fit to 30-40% of all new wearers and account for almost 60% of fits in Denmark. ³²

Mechanical differences between hydrogels and SiHys
Other notable differences between the properties of hydrogel and SiHy materials relate to their mechanical property differences. The incorporation of siloxane groups increases the modulus and stiffness of SiHys, ^{33, 48-50} resulting in a number of mechanical complications, ^51-53 particularly with lower water content SiHys, which have the highest modulus. ⁴⁹ These mechanical complications include superior epithelial arcuate lesions, ^54-56 mucin balls, ^57-61 epithelial flaps, ^62-65 contact lens associated papillary conjunctivitis 66-68 and corneal erosions. ^{52, 53} Another unanticipated issue relates to increased levels of corneal staining associated with the wear of certain SiHy materials when used with various preserved care systems. ^69-75 While the debate concerning the clinical relevance of this solution-induced corneal staining (SICS) rages on, ^76-80 the fact remains that for many subjects, refitting into hydrogel lenses and retaining the same solution eliminates the presence of SICS.

Silicone hydrogel vs. conventional hydrogel: complications and comfort
Data also exists concerning the relative rates of inflammatory and infective complications between these two categories of materials. When SiHy lenses were introduced to the market there was a hope that the rate of corneal infections associated with their overnight wear would decrease as a result of the increased oxygen available to the cornea. ⁸¹ However, studies indicate that despite the obvious reduction in hypoxia with SiHy materials, the risk of microbial keratitis is similar regardless of lens material. ^82-85 Several reports ^86-90 have demonstrated that wearing SiHy materials results in an almost two-fold increase in relative risk for infiltrative keratitis.

What about comfort advantages? Certainly clinicians see cases where wearers report increased comfort when switching from hydrogels to SiHy materials, but it is often difficult to remove any placebo affect whereby a wearer will report an improvement merely because they are being placed into ‘something new’. Scientific studies do exist of enhanced comfort with refits into SiHy materials, ^{3, 91, 92} but many studies lack the appropriate controls and were not undertaken as randomised clinical trials. Indeed, a recent review suggests that the introduction of SiHy materials has brought little in the way of an overall increase in comfort and that as a material group per se, substantial comfort advantages are hard to prove. ⁹³

On reflection, SiHy materials have brought many benefits associated with a reduction in hypoxic complications, but at the cost of reduced wettability and increased deposition in certain wearers, an unexpected increase in infiltrates and solution-related corneal staining, no reduction in microbial keratitis and a modest increase in comfort, at best. These realities have resulted in some practitioners and researchers questioning the widespread adoption of SiHy materials for all wearers. ^94-97 This is particularly the case for daily wear, where the oxygenation afforded the cornea appears adequate for the majority of wearers, particularly in low to moderate prescriptions. ^98-100

Revisiting hydrogels
For clinicians considering the option of ‘revisiting’ the use of hydrogels, the market has seen the recent introduction of two new daily disposable lenses that utilise ‘hydrogel’ technology, but from very differing viewpoints.

The first of these options is a ‘true’, traditional, conventional hydrogel lens from Bausch & Lomb, Biotrue ONEday (nesofilcon A; details in Table 1), which is manufactured from a new material termed a ‘HyperGel’. ^{97, 101} It is a copolymer of HEMA and N-vinyl pyrrolidone with a high water content (78%), a relatively low modulus (0.49MPa), is packaged in a borate buffer solution with poloxamine and incorporates a UV blocker. The lens undergoes very little dehydration when exposed to air and recent reports indicate that it is able to maintain its shape even in a dehydrated state. ^{97, 102}

As described above, one concern with high water content hydrogels relates to excessive dehydration and subsequent corneal staining, but this appears to not be an issue with this material. ¹⁰³ Another claim for this material is that the low level of dehydration results in a stable tear film over the front surface of the lens, and this maintains a high level of visual quality in patients who complain of blurry vision subsequent to lens dehydration, when undertaking tasks such as working at the computer, driving at night or being exposed to dry environments. ¹⁰⁴ To date, no published visual acuity data exists to support this claim, but modelling data ¹⁰⁵ and patient reports104 appear promising.

The second option is a silicone hydrogel daily disposable lens from Alcon, Dailies Total 1 (delefilcon A; details in Table 1), which is manufactured from a novel ‘water gradient’ material. ^106-108 The lens material is a silicone hydrogel with a core water content of 33%, a Dk of 140, central Dk/t of 156 for a -3.00D lens (the highest of any daily disposable lens on the market) and a core modulus of 0.7MPa. The surface has a modified hydrogel-like ‘gel’ with a water content that is >80% and is approximately 5-6µm thick. ^107-109 The surface gel increases towards the outermost surface, such that the water content approaches 100% at the lens/air interface, ¹¹⁰ resulting in a very soft surface with a very low modulus. ^{107-109, 111} Recent work has shown the surface modulus to be orders of magnitude lower than the core modulus, with a measured surface modulus of 0.014MPa. ¹¹² Given the growing importance of low friction and increased lubricity to increased comfort, ^113-116 one claimed advantage of this material is a very low coefficient of friction, which has been demonstrated in a number of reports. ^{111, 117}

The options available for practitioners considering daily disposables continues to grow, and these two novel lens materials offer practitioners who wish to consider the option of offering a ‘hydrogel-surface’ to their patients, for all the various potential issues described above, two excellent new opportunities. Clearly, the approaches used in the two materials are widely different. One is a true, conventional hydrogel and the other is a silicone hydrogel ‘wrapped in a hydrogel skin’. Two major differences between these classes of materials relate to water content and Dk/t, as shown in Figure 1, which provides these data for a variety of both hydrogel and silicone hydrogel daily disposable lenses. To date, very little independent data has been published on these new lenses and time will tell whether they live up to their promise and whether hydrogels can truly ‘rise like a phoenix’! The advantages of reusable SiHy materials have brought great benefits to many, but for some these new daily disposable options may well prove invaluable in the continuing search for ‘contact lens nirvana’.

Figure 1: Published water content and Dk/t for a variety of hydrogel daily disposables (green dots) and silicone hydrogels (red dots). As seen, hydrogels have higher water contents and lower Dk/t values compared with silicone hydrogels. The two materials discussed in this article have the highest Dk/t values for hydrogels (Biotrue ONEday) and silicone hydrogels (Dailies Total1), respectively.

Table 1: BioTrue 1-Day (nesofilcon)

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